1. Field of the Invention
The present invention relates to a digital image processing technique, and more particularly to a method and apparatus for registering images.
2. Description of the Related Art
Comparative analysis of medical images is typically performed to observe anatomical changes, to identify abnormal growth, to observe impact of treatment, etc. For example, comparison of mammograms may help identify abnormal structures and diagnose medical problems in breasts. Temporal comparison is an important tool in the analysis of mammograms. Temporal comparison is useful, for example, in cases where it is difficult to detect cancers without prior mammograms of the patient. In such cases, it is easier to detect cancers in current mammograms by comparing current mammograms with prior mammograms of the patient.
In analog screening mammography, temporal comparison is typically implemented by arranging mammograms side by side, or one above the other. A reader (for example, a radiologist) moves his head up and down or left and right to compare each region of interest (ROI) in current and prior mammograms. In the case of analog film mammograms, this manual comparison method is currently the best technique to perform a temporal comparison of mammograms. Major drawbacks, however, are associated with this method. For example, a large eye movement is needed to compare mammograms arranged side by side or one above the other. In addition, it is difficult to spot differences between mammograms arranged as such, and it is virtually impossible to hang more than 2 or 3 temporal cases (mammograms) for comparison, due to spatial constraints. Moreover, variations in breast positioning differences, mammogram background, etc., complicate the determination of meaningful differences between mammogram images.
While analog mammography has been gradually replaced by computed radiography (CR) and full field digital mammography (FFDM), hardcopy reading using light boxes has been replaced by softcopy reading using Picture Archiving and Communications Systems (PACS) and mammography workstations. Despite these advances, however, mammogram reading of images placed side by side is still extensively used by medical professionals because it is easily implemented.
Another method for temporal comparison of mammograms uses digital image processing to subtract a current mammogram image from a corresponding prior image. Temporal subtraction has been applied to temporal comparison of chest X-ray images, where it detects subtle differences between X-ray images. The temporal subtraction technique applied to chest X-ray images does not, however, work for mammograms, because unlike chest X-ray images, breast images are hard to align. This is so because breasts are deformable and contain no obvious landmarks, especially when they are compressed in a mammography machine. Hence, subtraction of breast images using the method that was applied to chest X-ray images does not produce medically significant results.
Disclosed embodiments of this application address these and other issues by performing comparison of anatomical images after registration of images. The methods and apparatuses of the present invention deform a prior image, for registration to a corresponding current image of an organ. After the prior and current images have been registered to each other, the images may be compared to detect differences between the organs illustrated in the images. In one embodiment, the images are breast images which are registered whereby one of the images is deformed by applying a registration to specific objects such as suspicious areas, dense tissue areas and fatty tissue areas, with constraints to preserve size and shape of the specific objects. The specific objects may be regions of interest in the breast images, such as cancerous lesions.